In coaters, the coating mix is applied to the paper or board web by means of an applicator device, and then the applied coating mix is smoothed to the desired thickness by means of a doctoring device. Conventionally, an excess amount of the coating mix is applied on the web at the coater, and in a plurality of different kinds of applicators a so-called machine circulation is used for treating the coating mix not adhering to the web. The machine circulation of the applicator and the excess coating mix doctored away from the web are returned to the storage tank of the coater, wherefrom the coating mix is again pumped to the coater. As the coating mix to be pumped to the coating station must be free from impurities, the machine circulation between the coating mix feed pump and the coating station must be provided with a pressure strainer to clean the coating mix to remove foreign material picked up from the web, paper pieces coming into the circulating coating mix from web breaks, and lumps possibly formed from coating mix aggregates. Such strainers chiefly comprise washable screens of the back-wash type or alternatively screens made from a wedge wire of triangular cross section and cleaned by mechanical scrapers. Today, the trend is away from back-wash screens, because the washing of the screen creates a great volume of coat-containing waste water that loads the wastewater cleaning system of the paper mill. Further, coating mix material losses in washable screens are greater than those of mechanically cleanable screens.
Mechanically cleanable screens are closed pressure vessels in which the coat is conveyed from a higher pressure to a lower pressure through the screening element. The screening element is made by coiling a continuous strand of triangular wire onto a cylinder, whereby the coat-passing gaps are formed between the adjacent turns of coiled wire. The width of the interwire gap is about 100-150 .mu.m and its length is very large, because a single, continuous gap will be formed by the length of wire wound on the cylindrical filter surface. This type of coil wire screen is cleaned by allowing a scraper element to pass over the cylindrical screen surface and the rejected material collected by the screen is removed at, e.g., 8 hour intervals by opening the rejects discharge valve. Herein, the rejects discharge valve is opened to discharge a few tens of liters (10-30 ) of the coating mix rejects that are next sewered to the waste treatment system of the paper mill. While pressure screens of this type have a large screening capacity per unit area of the screen surface, they are, due to the shape of the interwire gap, incapable of effectively separating foreign material of a long fibrous shape. As a result, the machine circulation of coating stations will gradually accumulate such fibrous particles whose sticking under the nip of doctor element in the coating station will cause striping of the applied coating mix, thus marring the produced paper web. Another nuisance caused by fibrous foreign material is the clogging of the screen gaps when the elongated fibers plug the screen mesh. Oblong fibers will orient themselves with the flow pattern, whereby they can either pass through the screen mesh or plug the mesh openings. Such long, fibrous foreign material will accumulate into the machine circulation of the coating mix from the coating mix return flow in the form fibers detached from the web being coated and of synthetic fibers which adhere to the web surface from the dryer felt and later become detached from web during the coating step. Since this kind of foreign material causes a significant load in maintenance and cleaning of the screens, its elimination from the machine circulation is of major importance. Mechanically cleaned pressure screens are still today handicapped by the large amount of coating mix rejects that increases the load of waste treatment equipment, since a single coater line conventionally includes a number of screens and from each of them must be discharged tens of liters of rejects into the sewer about three times a day.